1. Field of the Invention
Aspects of the present invention relate to a cooling plate used for cooling a fuel cell, and more particularly, to a cooling plate in which flow channels are improved to reduce temperature differences between portions of the cooling plate.
2. Description of the Related Art
A fuel cell is an electric generator that changes chemical energy of a fuel into electrical energy through a chemical reaction, and the fuel cell can continuously generate electricity as long as the fuel is supplied. FIG. 1 is a schematic drawing illustrating the energy transformation structure of a fuel cell. Referring to FIG. 1, when air that includes oxygen is supplied to a cathode 1 and a fuel containing hydrogen is supplied to an anode 3, electricity is generated as an electrolyte membrane 2 allows hydrogen ions to flow from the anode 3 to the cathode 1 through the electrolyte membrane 2 while electrons are forced to flow through a circuit, which produces usable energy. However, generally, a single unit cell 10 does not produce enough electricity to be useful. Therefore, as depicted in FIG. 2, electricity is generated by a fuel cell stack 20 in which a plurality of unit cells 10 is connected in series.
Accordingly, as depicted in FIG. 2, when hydrogen and oxygen are supplied through an endplate 21 of the fuel cell stack 20, corresponding materials are supplied to corresponding electrodes and circulated through each of the unit cells 10. Of course, as described above, hydrogen is supplied in the form of a chemical fuel, and oxygen is supplied in the form of air.
In the electrochemical reaction, heat is generated as well as electricity. Therefore, for a smooth operation of a fuel cell, the fuel cell must be continuously cooled by removing heat. For this purpose, in the fuel cell, as depicted in FIG. 2, a heat exchanger 30 is provided and a cooling plate 5 is mounted on every fifth to sixth unit cell 10 in the fuel cell stack 20. The cooling water flows from the heat exchanger 30 through a valve 50 to the fuel cell stack 20 where the cooling water absorbs heat. The cooling water absorbs heat in the fuel cell stack 20 while passing through flow channels 5a (refer to FIG. 3) of the cooling plate. The cooling water returns 40, after having absorbed heat from the fuel cell stack 20 to the heat exchanger 30 to release the absorbed heat before returning to the fuel cell stack 20 again.
Referring to FIG. 3, in each of the unit cells 10 stacked in the fuel cell stack 20, a bipolar plate 4 includes surface flow channels 4a for supplying and recovering hydrogen or oxygen to and from the cathode 1 and the anode 3, and the surface flow channels 4a are connected to each other. The bipolar plate 4 is configured about a polymer electrolyte membrane 2 that only allows positively charged ions to pass. The cooling plate 5 is illustrated as being adjacent to the bipolar plate 4 and polymer electrolyte membrane 2. The cooling water flows into the cooling plate 5 through a plurality of flow channels 5a, absorbing heat, and then back out of the cooling plate 5.
However, if the temperatures of portions of the cooling plate as depicted in FIG. 4 are measured during the cooling process a central portion 5a-2 of the cooling plate 5 where the heat absorption action is the highest has the highest temperature, and an inlet 5a-1 and an outlet 5a-3, where the cooling water enters and leaves, have lower temperatures. This temperature distribution is natural because after cooling water cooled at the heat exchanger 30 enters through the inlet 5a-1, the cooling water absorbs a large amount of heat while passing through the central portion 5a-2, and a small amount of heat dissipates from the cooling water after passing the central portion 5a-2 prior to leaving through the outlet 5a-3. However, having a large temperature difference between the central portion and the both end portions causes problems with regard to generated current density. According to the actual temperature measurement results, as depicted in FIG. 4, the temperature in the central portion of the cooling plate 5 reaches approximately 150° C., the temperature at the inlet 5a-1 reaches approximately 135° C., and the temperature at the outlet reaches approximately 140° C. That is, a temperature difference of 10-15° C. is generated in a single cooling plate 5. When the temperature difference is large, there is a possibility of deformation of the cooling plate 5 due to thermal stress. Above all, the large temperature difference can adversely affect the electrochemical reaction of the adjacent unit cells 10. When the temperature difference is large, resistance in portions of the electrolyte membrane 2 in the unit cells 10 varies resulting in large current density deviations, thereby generating electricity having a non-stable voltage.
Accordingly, in order to solve this problem, there is a need to develop a new type of cooling plate that can minimize the temperature difference in a single cooling plate 5 of a corresponding unit cell 10.